The good:
Tokyo Marui
KSC
Maruzen
Tanaka
TOP
Western Arms
Marushin
Sun Project
The OK:
Airsoft Elite
Classic Army
(those 2 can be good, quality control leaves a lot to be desired. these are good bets, but I really don't recommend them unless you know what you are doing)
New MGC
HFC
KJ Works
The crap:
STTI
KWC
UHC
Academy
Y&P
The Elite (the Ferraris of airsoft):
Digicon
Asahi
Sheriff
I'm sure I missed some. Again there are some exceptions. KWC makes a couple of good guns, while Tokyo Marui has some crappy stuff (Sig series comes to mind). Almost all of the good brands are based in Japan. The crap stuff comes from Taiwan. Basically avoid anything from Taiwan (I think Tanaka is out of Taiwan and is the exception).
The TMs are basically the workhorses of airsoft. 90% of players will have a TM. Very simple, very effective, very cost efficient. Parts are easy to get and repairs are easy as most folks know how to repair them.
AEGs use batteries and motors. Usually 600-1000 rounds per minute. Easy to use, very little maintenance. Just charge the battery, fill a mag, and you're good to go. You can go through 10's of thousands of rounds before you'll have major work to do (replacing a motor for example). Some of the guns get more complex like the TOP. There is not case for the parts to fit in, they are physically attached to the gun so they are hard harder to work with.
Gas guns are my favorite. Most fire like real guns. You pull the slide to chamber a round, pull the hammer back, flip the safety, shoot. The slide cycles with each shot, and when you are out the slide catches back. Kick about half as much as a .22 depending on what you use (something like a Digicon will be equivelant to a .22). Just a very fun thing to shoot. Especially the full autos. I'm gonna pick up an M-11 soon, should be a blast. Most pistols are semi-auto (some full) and there are full auto subs like the MP5K from Maruzen, or the Stery TMP. They even make shotguns that are gas, very very cool. There's just something about them. Much more problem prone than AEGs. You get less power in the winter too (colder weather and the gas doesn't expand as much). May have to replace an o-ring eventually. Some autos have a cool down where they lose power if you hold down the trigger to long (magazine gets cold where the gas is stored). Definately the coolest things around.
Springers are pretty much suck. Unless you are getting a sniper rifle like the ASP2 or M40, stay away. Very low power, have to cock it for every shot. Basically junk if it's a pistol. There are some cool rifles though like the sniper ones, and most shotguns are springers. I personally like the gas shotguns, but they are more difficult to keep in good shape. Springers are extremely simple. Anyone can take them apart and put them back together.
Personally, I think the best thing you can do is save up and get a TM AEG (something in the MP5 or M16 series) and a gas pistol like an M9 from one of the other Japanese brands. It'll be soooooo worth it. You get the benefit of pissing bb's at 800 rpm, and the coolness of a gas pistol.